Request for Feedback: `flagged_pointer` - A zero-cost abstraction for storing flags in a pointer's unused bits

code review
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Hello, everyone!

I have recently developed a new crate, flagged_pointer, which aims to store additional flag information within the unused bits of aligned pointers.

Github: flagged_ptr
Crate: flagged_ptr

My code relies on bitwise operations on pointers, so I used unsafe code extensively. I am seeking a delicate code review for safety, and any other feedback is welcome too!

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  • I suggest you test your code with cargo +nightly miri test. Perhaps you have done so already, but it isn’t done in your GitHub CI. Testing with Miri is an important step that is highly valuable for non-FFI unsafe code. Miri cannot check that your code is sound, but it can check that for the tests that you do have, it is not executing UB.

  • Glancing at your code, it seems that you are using a NonZeroUsize value for storing the modified pointer. This is problematic, because it discards provenance. You should use a pointer type to store any pointer you are going to dereference later. It’s okay to do arithmetic on the address of that pointer.

8 Likes
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Thank you so much for your feedback! I've fixed the issues you pointed out and have published v0.1.1 of the crate:

  • All pointers now have their provenance (by using the with_addr method).
  • Added a Miri test.

Let me know what you think!

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